Essential Cell Biology 5th edition

Mendel and the Laws of Inheritance
667
Figure 19−23 Parent plants produce gametes that each contain
one allele for each trait; the phenotype of the offspring depends
on which combination of alleles it receives. Here we see both the
genotype and phenotype of the pea plants that were bred in the
experiments illustrated in Figures 19−21 and 19−22. The true-breeding
yellow-pea plants produce only Y-bearing gametes; the true-breeding
green plants produce only y gametes. The F 1 offspring of a cross
between these parents all produce yellow peas, and they have the
genotype Yy. When these hybrid plants are bred with each other, 75%
of the offspring have yellow peas, 25% have green. The gray box at
the bottom, called a Punnett square after a British mathematician who
was a follower of Mendel, allows one to track the segregation of alleles
during gamete formation and to predict the outcomes of breeding
experiments like the one outlined in Figure 19−22. According to the
system established by Mendel, capital letters indicate a dominant
allele and lowercase letters a recessive allele.
with yellow peas will produce two classes of gametes: half the gametes
will get a yellow-pea allele and half will get a green-pea allele. When the
hybrid plants self-pollinate, these two classes of gametes will unite at
random. Thus, four different combinations of alleles can come together in
the F 2 offspring (Figure 19−23). One-quarter of the F 2 plants will receive
two alleles specifying green peas; these plants will produce green peas.
One-quarter of the plants will receive two yellow-pea alleles and will
produce yellow peas. But one-half of the plants will inherit one allele for
yellow peas and one allele for green. Because the yellow allele is dominant,
these plants—like their heterozygous F 1 parents—will all produce
yellow peas. All told, three-quarters of the offspring will have yellow peas
and one-quarter will have green peas. Thus Mendel’s law of segregation
explains the 3:1 ratio that he observed in the F 2 generation.
Mendel’s Law of Segregation Applies to All Sexually
Reproducing Organisms
Mendel’s law of segregation explained the data for every trait he examined
in pea plants, and he replicated his basic findings with corn and
beans. But his rules governing inheritance are not limited to plants: they
apply to all sexually reproducing organisms (Figure 19−24).
Consider a phenotype in humans that reflects the action of a single gene.
The major form of albinism—Type II albinism—is a rare condition that
is inherited in a recessive manner in many animals, including humans.
Like the pea plants that produce green seeds, albinos are homozygous
recessive: their genotype is aa. The dominant allele of the gene (denoted
A) encodes an enzyme involved in making melanin, the pigment responsible
for most of the brown and black color present in hair, skin, and the
phenotype: yellow pea
genotype:YY
Y
FEMALE
GAMETES
PARENTAL GENERATION
gametes
25% YY
25%
phenotype: green pea
genotype:
CROSS-
FERTILIZATION
FIRST GENERATION (F 1 )
phenotype: yellow pea
genotype: Y
Y
SELF-
FERTILIZATION
25% Y 25% Y
F 2 GENERATION
ECB5 e19.23/19.23
gametes
Y
MALE
GAMETES
Figure 19−24 Mendel’s law of
segregation applies to all sexually
reproducing organisms. Dogs are bred
specifically to enhance certain phenotypic
traits, including a diverse range of body
size, coat color, head shape, snout length,
ear position, and fur patterns. Scientists
have been conducting genetic analyses
on scores of dog breeds to search for the
alleles that underlie these common canine
characteristics. A single growth factor
gene has been linked to body size, and
three additional genes have been shown
to account for coat length, curliness, and
the presence or absence of “furnishings”—
bushy eyebrows and beards—in almost
all dog breeds. (By Ester Inbar, available
from http://commons.wikimedia.org/wiki/
User:ST.)